The present invention is directed to a simplified low cost counter assembly. In a specific application it has use in electrostatographic printing apparatus and in particular, it provides an indication that a removable toner cartridge is about to or has exhausted its useful life.
Recently, there has been a tendency in the design of electrostatographic printing apparatus to place one or more of the functional units such as a photoconductor drum or developer housing in a removable processing cartridge or unit so that the customer or operator of the printing machine may replace a functional unit when its lifetime has been exhausted or a different processing parameter such as toner color is desired. Exemplary of such machines are those described and illustrated, for example, in U.S. Pat. Nos. 3,985,436 to Tanaka et al., 4,556,308 to Hoppner et al. and 5,111,246 to Brailsford et al., all of which are hereby incorporated in their entirety herein by reference. Accordingly, the removable processing cartridges may be designed to contain a photoreceptor, a developing device, a cleaning device, as well as a charging device. Alternatively, instead of placing all of the functional elements in a single processing cartridge, there may be two or more processing cartridges wherein, for example, the photoreceptor, cleaning device and charge device are in one removable processing cartridge while the developer device is in another removable processing cartridge.
There is a desire for the operator to know when the end of the functional lifetime of a removable processing unit is approaching and when it has finally approached. This is true whether the functional unit comprises a photoreceptor, a developer housing, or indeed is a toner cartridge. It is particularly important to the operator to have some warning that the supply of toner or developer to the developer housing from a toner cartridge is about to be exhausted or in fact is exhausted.